There are several ways for a device such as a mobile phone or tablet to determine its location. Conventional techniques include the use of a satellite-based positioning system such as GPS (Global Positioning System), or trilateration based on signals communicated between the device and a plurality of base stations of a cellular network.
A further technique is described in WO 2013/016439. According to this, a plurality of light sources in a given location each broadcast a respective identifier in the form of a coded light signal modulated into the light emitted from that light source. A device such as a mobile phone can then use its in-built camera to read the identifier coded into the light from such a source, and this identifier can be used to look up coordinates of the light source. Thus the device can determine that it is roughly in the vicinity of this light source's coordinates. When multiple sources appear in the field of view of a camera, with all their identifiers known, then a form of triangulation can be performed to determine a more fine-grained location of the camera, and thereby the device.